potterton boiler clicking noises when boiler is off?

hi, we have problem with a potterton boiler which appears to be related to the release of hot water when the boiler goes to off.

We have so far replaced the PCB, checked the fresh air vent to the boiler area (OK), replaced the gas valve on the boiler, replaced the airswitch and now finally replaced the connected pump on the CH system.
But we are still getting a 'clicking' from the PCb to the pump after the boiler goes off. As i understand it, this is the boiler trying get rid of the hot water and sending a signal to the pump to run and therefore circulate the water in the system.
The problem is this clicking goes on and off for about 1/2 hour and is annoying and quite loud. Presumably the pump trying to clear the hot water and then stopping and then starting again..

has anyone any ideas or experienced anything similar?


thanks

toby